Method and system for generating a complex pseudonoise sequence for processing a code division multiple access signal

ABSTRACT

In a wireless communication system, a chip time is selected in a complex pseudonoise (PN) sequence generator. For a next chip time following the selected chip time, a phase difference between a previous complex PN chip and a next complex PN chip is restricted to a preselected phase angle. In one embodiment, every other chip time is selected and the preselected angle is 90 degrees.

FIELD OF THE INVENTION

The present invention is related in general to wireless communicationsystems, and more particularly to a method and system for processingcode division multiple access signals with a complex pseudonoisesequence.

BACKGROUND OF THE INVENTION

In power amplifiers used to transmit modulated radio frequency signals,is desirable to operate with an input signal having a lowpeak-to-average ratio. Signals with high peak-to-average ratios areundesirable because the power amplifier produces extraneous side bandswhen a peaking signal causes it to operated in a nonlinear portion ofits operating range. These extraneous side bands are produced by amechanisms called AM-to-PM conversion and AM-to-AM conversion whenpassing a signal with large amplitude fluctuations. Furthermore, theseside bands deprive the information signals of some of their portion ofthe transponder power, and also can interfere with nearby channels(adjacent channel interference).

In a communications system using quartenary phase shift keying (QPSK)the signal phase can be any of one of four phases for the duration ofeach phase shift interval. This is shown in the signal space diagram inFIG. 1, wherein phase 30 illustrates the phase of constellation point32, which is one of the constellation points 32-38. Transitions 40-46illustrate the permitted phase changes between phase shift intervals. Azero degree transition is shown at reference numeral 40. Examples of π/2radians or 90° transitions are shown at reference numerals 42 and 44,and a 180° or π radian transition is shown at reference numeral 46.

In a code division multiple access (CDMA) system, such as a CDMA systemimplemented according to American National Standards Institute (ANSI)J-STD-008, user data is spread and modulated by a pseudorandom noise(PN) sequence, which is periodic and has noise-like properties. Forexample, with reference to FIG. 2, in direct sequence QPSK transmitter60, real-valued user data 62 is split and multiplied by 2 PN sequences:a PN_(I) sequence 64 and a PN_(Q) sequence 66, using multipliers 68 and70, respectively. The PN sequences are generated by PN_(I) and PN_(Q)sequence generators 72 and 74, respectively. The duration of the outputof these PN sequence generators may be referred to as a chip time orchip interval, which is the duration of a single pulse in a directsequence modulated signal.

After in-phase (I) and quadrature (Q) components of user data 62 havebeen multiplied by PN_(I) sequence 64 and PN_(Q) sequence 66, thesignals output by multipliers 68 and 70 are each separately filtered bypulse shaping filters 76. Pulse shaping filters 76 may be implementedwith finite impulse response filters that filter higher frequencycomponents from the signal.

Next, the filtered I and Q signal components are multiplied byquadrature carrier components 78 and 80 using multipliers 82 to produceI and Q radio frequency (RF) signals 84 and 86. Signals 84 and 86 arethen added together in summer 88. The output of summer 88 is RFmodulated signal 90, which is then amplified by power amplifier 92. Theoutput of power amplifier 92 is then coupled to antenna 94 fortransmitting the signal to a receiving unit.

As shown in FIG. 2, PN sequence generators 72 and 74 are typicallyimplemented with a maximal-length linear feedback N-bit shift register,wherein selected stages are tapped and exclusive ORed with the shiftregister output to form a signal that is fedback to the shift registerinput. Other ways of implementing PN sequence generators may be used.For example, nonlinear feedback shift registers may be used to generatethe PN sequences.

A combination of the outputs of PN_(I) and PN_(Q) generators 72 and 74may be referred to as having a complex value that corresponds to aphase. For example, referring again to FIG. 1, if PN_(I) equals 1 andPN_(Q) equals 1 the complex PN value of (1, 1) corresponds to phase 30,which is π/4 radians. Other values output by the complex PN generatorcorrespond to constellation points 34-38. Transitions 40-46 from oneconstellation point to another are determined by the difference betweena previous complex PN chip and a next complex PN chip generated by thecomplex PN sequence generator in the next chip time.

When RF modulated signal 90 peaks and causes power amplifier 92 tooperate in a non-linear region, extraneous side bands are created in thetransmitted signal. These side band signals may be eliminated byreducing the occurrence of peaks in RF modulated signal 90, hence thedesirability of reducing the peak-to-average ratio.

Peaks in RF modulated signal 90 occur as a result of receiving asequence of chip values in pulse shaping filter 76 that highlycorrelates with the impulse response of pulse shaping filter 76.Furthermore, the peaking of signal 90 is greater when peaks are formedin pulse shaping filters 76 in both the I and Q channels at the sametime.

In the prior art, π/2 BPSK modulation has been used to reduce thepeak-to-average in signals sent to the power amplifier. However, π/2BPSK modulation produces BPSK spreading, which is inferior becausesignals from other users are not easily rejected.

QPSK spreading, on the other hand, provides superior rejection betweenuser's signals, but produces a signal with an inferior peak-to-averageratio. For a more detailed discussion regarding spreading methods, seethe book “CDMA, Principles of Spread Spectrum Communications,” by AndrewJ. Viterbi, published by Addison Wesley in 1995, pages 26-32.

Thus, it should be apparent that a need exists for an improved methodand system for generating a complex pseudonoise sequence for processinga code division multiple access signal wherein the complex pseudonoisesequence helps reduce the peak-to-average ratio of a modulatedcommunications signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the invention are setforth in the appended claims. The invention itself, however, as well asa preferred mode of use, further objects, and advantages thereof, willbest be understood by reference to the following detailed description ofan illustrative embodiment when read in conjunction with theaccompanying drawings, wherein:

FIG. 1 depicts a QPSK signal space diagram according to the according tothe prior art;

FIG. 2 is a direct sequence spread spectrum modulator in accordance withthe method and system of the prior art;

FIG. 3 is a direct sequence spread spectrum modulator incorporating amethod and system for generating a complex pseudonoise sequence inaccordance with the method and system of the present invention; and

FIG. 4 is a high-level logic flowchart which illustrates the method andsystem of generating a complex pseudonoise sequence according to thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

With reference now to the figures, and in particular with reference toFIG. 3, there is depicted a direct sequence spread spectrum transmitterincorporating the method and system for generating a complex pseudonoisesequence in accordance with the method and system of the presentinvention. As illustrated, direct sequence QPSK transmitter 110 receivesreal-valued user data 62, which is split and multiplied by two PNsequences generated according to the present invention. While generationof PN_(I) and PN_(Q) sequences 112 and 114 is new according to thepresent invention, many remaining portions of the transmitter operate inthe manner discussed above. For example, multipliers 68 and 70 operatein much the same way as described with reference to FIG. 2. Similarly,pulse shaping filters 76 are used to filter high frequencies componentsfrom signals output from multipliers 68 and 70. The I and Q signals arethen modulated in multipliers 82 by quadrature carrier components 78 and80. I and Q RF signals 116 and 118 are added together in summer 88 toproduce RF modulated signal 120, which is then amplified by poweramplifier 92 and coupled to antenna 94 for transmitting the signal to areceiving unit. Note that signals 116, 118, and 120 are new because theyare modified according to the present invention using the new complex PNsequence.

In a preferred embodiment, the generation of the improved complexpseudonoise sequence begins with C₁ and C₂ sequence generators 130 and132, which may be implemented in much the same manner as PN_(I) andPN_(Q) sequence generators 72 and 74 shown in FIG. 2. The outputs ofsequence generators 130 and 132 have values C₁ and C₂ during any givenchip time. Signals C₁ and C₂ are both coupled to last phase register134, and inputs to one side of multiplexer 136. Last phase register 134converts the values of C₁ and C₂ into a phase angle and stores such aphase angle for one chip time.

Last phase information output from last phase register 134 is coupled tophase adjuster 138, which also receives the current chip value of C₁from PN_(I) sequence generator 130. As shown in FIG. 4, phase adjuster138 is a ±90 degree phase adjuster wherein the determination of whetherto add or subtract 90 degrees depends upon the current value of C₁. Inone implementation of phase adjuster 138, the sign of either C₁ or C₂,which are inferred from the phase input from last phase register 134, ischanged depending upon whether the current value of C₁ is a +1 or a −1.Phase adding or phase subtracting in phase adjuster 138 may becontrolled according to any sequence that may be determined or preset inthe receiver.

The outputs of phase adjuster 138, PN_(I) and PN_(Q), are coupled toinputs of multiplexer 136, as shown.

The values output from multiplexer 136 are selected from the pairs ofinputs based upon a signal from chip selector 140. Chip selector 140 isclocked by a clock signal that is common to both PN_(I) sequencegenerator 130 and PN_(Q) sequence generator 132, wherein the period ofthe clock is a chip time. In a preferred embodiment, chip selector 140causes multiplexer 136 to select the output of phase adjuster 138 duringevery other chip time. When the output of phase adjuster 138 is notselected, the unmodified, current values of C₁ and C₂ are output frommultiplexer 136. Thus, in the preferred embodiment, at every other chiptime, the phase of the next complex PN chip differs from the phase ofthe previous complex PN chip by 90 degrees.

PN_(I) and PN_(Q) sequences 112 and 114, which are the outputs ofmultiplexer 136, are coupled to multipliers 68 and 70, respectively, andare thereby used to process or spread a code division multiple accesssignal that carries user data 62.

With reference now to FIG. 4, there is depicted a high-level logicflowchart that illustrates the method of generating a complexpseudonoise sequence according to the present invention. As illustrated,the process begins at block 200 and thereafter passes to block 202wherein the process stores a current PN chip phase. This may beimplemented by converting the current values of C₁ and C₂ to a phase,wherein C₁ and C₂ have values of +/−1.

Next, the process determines whether or not a phase change for a nextchip should be restricted to a predetermined angle, as depicted at block204. If the next chip is not selected as a chip for which the phasechange will be restricted, the process reads C₁ and C₂ from the outputsof the complex PN sequence generator, as illustrated at block 206. Theprocess then equates PN_(I) with C₁ and PN_(Q) with C₂, as illustratedat block 208. Finally, the process outputs the PN_(I) and PN_(Q) values,as depicted at block 210. Because the process had not selected this chiptime to restrict the phase change of the next PN chip, the PN_(I) andPN_(Q) values are output as the next PN chip without modification.

With reference again to block 204, if the next chip is selected forrestricting the phase change, the process recalls the last PN chipphase, as illustrated at block 212. Next, the process examines code C₁and determines whether or not it is equal to 1, as depicted at block214. If C₁ is equal to 1, the process adds 90 degrees to the last PNchip phase to compute the next PN chip phase, as illustrated at block216. However, if code 1 is not equal to 1, the process subtracts 90degrees from the last PN chip phase to compute the next PN chip phase,as depicted at block 218.

After adding or subtracting 90 degrees from the last PN chip phase tocompute the next PN chip phase, the process converts the next PN chipphase to PN_(I) and PN_(Q) values, as illustrated at block 220.Thereafter, the PN_(I) and PN_(Q) values are output, as depicted atblock 210. The process then iteratively returns to block 202, whereinthe current PN chip phase is stored.

While the present invention generates a complex PN sequence used toprocess or spread a CDMA signal in a transmitter, this method and systemfor generating the complex PN sequence must also be used in a receivingunit to process or despread the received CDMA signal. Therefore, thosepersons skilled in the art should recognize that CDMA receivers mustalso practice the method and system of the present invention.

The present invention has been described in reference to a system thattransmits real user data 62. Persons skilled in the art should recognizethat user data may be complex data and that multipliers 68 and 70 may beimplemented in a complex manner.

Those persons skilled in the art should recognize that the spreadingscheme that uses the complex PN generator of the present invention isneither a QPSK spreading scheme nor a π/2 BPSK spreading scheme; thespreading scheme produced by using the present invention is a hybridwherein selected chip times behave like a π/2 BPSK spreading scheme andthe remaining chip times behave like a QPSK spreading scheme. Thishybrid spreading scheme avoids the low interference rejection of the π/2BPSK spreading and avoids the high peak-to-average ratio of the QPSKspreading.

The foregoing description of a preferred embodiment of the invention hasbeen presented for the purpose of illustration and description. It isnot intended to be exhaustive or to limit the invention to the preciseform disclosed. Modifications or variations are possible in light of theabove teachings. The embodiment was chosen and described to provide thebest illustration of the principles of the invention and its practicalapplication, and to enable one of ordinary skill in the art to utilizethe invention in various embodiments and with various modifications asare suited to the particular use contemplated. All such modificationsand variations are within the scope of the invention as determined bythe appended claims when interpreted in accordance with the breadth towhich they are fairly, legally, and equitably entitled.

We claim:
 1. A method in a wireless communication system for generatinga complex pseudonoise (PN) sequence for processing a code divisionmultiple access signal, the method comprising the steps of: selecting achip time in a complex PN sequence generator; and at each selected chiptime, restricting a phase difference between a previous complex PN chipand a next complex PN chip to a preselected phase angle.
 2. The methodfor generating a complex pseudonoise sequence according to claim 1wherein the step of selecting a chip time in a complex PN sequencegenerator further includes periodically selecting every Nth chip time ina complex PN sequence generator.
 3. The method for generating a complexpseudonoise sequence according to claim 2 wherein N equals 2 forselecting every other chip time in the complex PN sequence generator. 4.The method for generating a complex pseudonoise sequence according toclaim 1 wherein the step of restricting a phase difference between aprevious complex PN chip and a next complex PN chip to a preselectedphase angle further includes restricting a phase difference between aprevious complex PN chip and a next complex PN chip to 90°.
 5. Themethod for generating a complex pseudonoise sequence according to claim1 wherein the step of restricting a phase difference between a previouscomplex PN chip and a next complex PN chip to a preselected phase anglefurther includes adding 90° or subtracting 90° from a phase of aprevious complex PN chip to produce a next complex PN chip.
 6. Themethod for generating a complex pseudonoise sequence according to claim5 wherein the step of adding 90° to or subtracting 90° from a phase of aprevious complex PN chip to produce a next complex PN chip furtherincludes adding 90° to or subtracting 90° from a phase of a previouscomplex PN chip in response to a value of a previous complex chip toproduce a next complex PN chip.
 7. The method for generating a complexpseudonoise sequence according to claim 5 wherein the step of adding 90°to or subtracting 90° from a phase of a previous complex PN chip toproduce a next complex PN chip further includes adding 90° to orsubtracting 90° from a phase of a previous complex PN chip according toa preselected sequence to produce a next complex PN chip.
 8. The methodfor generating a complex pseudonoise sequence according to claim 1wherein the step of periodically selecting a chip time in a complex PNsequence generator further includes periodically selecting N chip timeswithin a series of M consecutive chip times in a complex PN sequencegenerator.
 9. A system in a wireless communication system for generatinga complex pseudonoise (PN) sequence for processing a code divisionmultiple access signal, the system comprising: means for selecting achip time in a complex PN sequence generator; and means for restrictinga phase difference between a previous complex PN chip and a next complexPN chip to a preselected phase angle at each selected chip time.
 10. Thesystem for generating a complex pseudonoise sequence according to claim9 wherein the means for selecting a chip time in a complex PN sequencegenerator further includes means for periodically selecting every Nthchip time in a complex PN sequence generator.
 11. The system forgenerating a complex pseudonoise sequence according to claim 10 whereinN equals 2 for selecting every other chip time in the complex PNsequence generator.
 12. The system for generating a complex pseudonoisesequence according to claim 9 wherein the means for restricting a phasedifference between a previous complex PN chip and a next complex PN chipto a preselected phase angle further includes means for restricting aphase difference between a previous complex PN chip and a next complexPN chip to 90°.
 13. The system for generating a complex pseudonoisesequence according to claim 9 wherein the means for restricting a phasedifference between a previous complex PN chip and a next complex PN chipto a preselected phase angle further includes means for adding 90° to orsubtracting 90° from a phase of a previous complex PN chip to produce anext complex PN chip.
 14. The system for generating a complexpseudonoise sequence according to claim 13 wherein the means for adding90° to or subtracting 90° from a phase of a previous complex PN chip toproduce a next complex PN chip further includes means for adding 90° toor subtracting 90° from a phase of a previous complex PN chip inresponse to a value of a previous complex chip to produce a next complexPN chip.
 15. The system for generating a complex pseudonoise sequenceaccording to claim 13 wherein the means for adding 90° to or subtracting90° from a phase of a previous complex PN chip to produce a next complexPN chip further includes means for adding 90° to or subtracting 90° froma phase of a previous complex PN chip according to a preselectedsequence to produce a next complex PN chip.
 16. The system forgenerating a complex pseudonoise sequence according to claim 9 whereinthe means for periodically selecting a chip time in a complex PNsequence generator further includes means for periodically selecting Nchip times within a series of M consecutive chip times in a complex PNsequence generator.